The present invention is directed to improvement in cellular retroreflective sheeting of the type first taught in U.S. Pat. No. 3,190,178. Such sheeting comprises a base sheet in which a layer of transparent microspheres is partially embedded and partially exposed, a transparent cover film disposed in spaced relation above the layer of microspheres, and a network of narrow intersecting bonds that adhere the base sheet and cover film together and divide the space between the base sheet and cover film into hermetically sealed cells. A continuing effort with such sheeting is to increase the resistance of the sheeting to delamination of the cover film from the base sheet. Upon such delamination, moisture can enter the cells and cover the exposed faces of the microspheres, whereupon the focusing capacity of the microspheres is reduced and destructive corrosion of a specularly reflective layer behind the microspheres is promoted.
Significant increases in the bond strength between the cover film and base sheet were achieved by the invention taught in U.S. Pat. No. 4,025,159, in which the bonds are cured in situ after being thermoformed into place. Commercial sheeting using such bonds has displaced previous versions and won wide acceptance.